Journal of Ceramic Processing Research. Vol. 16, No. 5, pp. 495~498 (2015)

495

J O U R N A L O F

CeramicProcessing Research

Electrical properties of Dy-doped BaTiO3-based Ceramics for MLCC

Seung-Hwan Leea,b, Dae-Young Kima, Min-Kee Kima, Hong-Ki Kimb, Jeong Hyun Leeb, Esther Baekb

and Jung-Rag Yoona,*

aR&D Center, Samwha Capacitor, Young-In, KoreabDept. of Electronic Materials Engineering, Kwangwoon University, Seoul, Korea

To understand the contribution of rare earth additive material (Dy2O3) in BaTiO3-MgO-Mn3O4-BaCO3-SiO2-xwt%Dy2O3-based (hereafter BT-based) ceramics (0.3≤ × ≤1.2) for multilayer ceramic capacitors (hereafter MLCC) prepared using BT-based powder and their properties were investigated electrical and structural properties. The BT grain size and dielectricconstant of Dy-doped samples was found to increase with increasing Dy2O3 content. In particular, the 0.6 wt%Dy2O3 dopedsample shows high resistance capacitance (RC) values of 2500 Ω • F, dielectric constant of 3600, dielectric loss of 1.4%, stabletemperature coefficient of capacitance (TCC). These results indicate that 0.6 wt%Dy2O3 doped BT-based ceramic is apromising candidate material for superior X5R type MLCC.

Key words: Dy2O3, BaTiO3-based,TCC, X5R type, MLCC.

Introduction

Multilayer ceramic capacitors (MLCCs) are widelyused in the electronic industry for smart phone,automotive applications and other applications [1, 2].There has been a continuous scaling down of both thedielectric and electrode layer thickness for highvolumetric capacitance [3-5]. The BaTiO3 based ceramicswith Ni internal electrodes must be cofired under highlyreducing atmospheres to prevent Ni oxidation. The co-firing of BaTiO3 oxide and Ni metal requires a reducingatmosphere, and thus oxygen vacancies (VO) aregenerated in BaTiO3 insulating layers. It was shown thatVO in BaTiO3 layers spoils the insulating reliability ofMLCC, i.e., VO migrates the vicinity of electrodes underdirect-current electric field, and then it finally results in ashort circuit. The temperature stability of capacitancecan be created by control of the so-called core shellstructure. The core shell structures are the coexistenceof ferroelectric structure of tetragonal BaTiO3 andferroelectric region by a solid solution of rare oxides inBaTiO3. In a previous study, It was suggested that thesubstitution of rare earth elements into the shell phaseof the BaTiO3-MgO-R2O3(such as Dy, Ho, Er, Y) basesystem changed from Ba-site (act as a donor) to Ti-sites(act as an acceptor) occupation, rare earth elementswere effected forming core-shell microstructures andhigh insulation resistance and improved temperaturecharacteristics and reliability [6]. In general, the

sintering condition such as soak time and temperature,atmosphere are of significance for the microstructure ofBaTiO3, the physical and chemical properties of startingmaterials could also have an effected MLCC, especially tothe tiny amount of additives. For example, the additives,such as rare earth oxide, influenced the formation of thecore shell structure and dielectric properties. This paperdescribes the effect of the dielectric properties of BT-basedceramics with various amount of Dy2O3.

Experiments

Conventional mixed oxide processing was used toprepare the BT-based ceramics with different Dy2O3

(X = 0.3. 0.6, 0.9, 1.2 mol%) contents. BaTiO3 powder(Kyriz, Japan) with a particle size of 200 nm was used asthe raw material. The BaTiO3 powder and additives suchas MgO (Kojundo chemical, Japan), BaCO3(Sakai,Japan), Dy2O3 (Rhodia, China), SiO2 (Kojundo chemical,Japan) and Mn3O4 (Kojundo chemical, Japan), were ball-milled in ethanol with zirconia balls for 24 h and thengranulated with polyvinyl alcohol as a binder. Thegranulation was then pressed into ceramic disks, 16 mmin diameter, at 1,500 kg/cm2. The green disks weresintered at the optimum sintering temperatures of 1200-1280 oC for 2 hours in a reducing atmosphere. The BT-based powders with organic binder system andmethylethly-ketone (MEK) and toluene as solvent werecast into green sheet by doctor blade. After printing thenickel sheets as internal electrode, MLCC was laminated,pressed, and then cut into small green chips. The greenchips were held at 260 oC for 40 h in an air atmosphereto burn out the binder and then sintered at 1230 oC for2 h. After grinding the sintered chips in a barrel, MLCCs

*Corresponding author: Tel : +82-31-330-5922Fax: +82-31-332-7661E-mail: yoonjungrag@samwha.com

496 Seung-Hwan Lee, Dae-Young Kim, Min-Kee Kim, Hong-Ki Kim, Jeong Hyun Lee, Esther Baek and Jung-Rag Yoon

were terminated with Cu electrode paste. Then, Ni andSn were electroplated in regular sequence to the endterminals. The microstructure of sintered MLCC sampleswas characterized by scanning electron microscopy(SEM, S-4700, Hitachi, Japan). The x-ray diffraction(XRD) patterns were measured on Rigaku Corporation,Rigaku Model SmartLab installed at the Hanyang Centerfor Research Facilities (Seoul) at Hanyang University.The TCC curves and capacitances were determined usingHP 4284A precision LCR meter (HP, USA) at testvoltage of 1 V and a frequency of 1 kHz. Capacitancemeasurements were taken in the temperature range from−55 oC to 125 oC placing the MLCCs in a temperaturechamber (S&A Inc., USA). The insulation resistancewas measured using high resistance meter (HP 4329).

The high temperature insulation resistance of MLCCwas performed using HP4329 at 125 oC under stepwiseboosting voltage from 1 Vr ~ 7.5 Vr (1 Vr = 35 V).

Results and Discussion

Fig. 1 shows the dielectric constant and loss of BT

based ceramics as a function of Dy2O3 content andsintering temperature. The dielectric properties of Dydoped BT-based MLCC tend to depend on the sinteringtemperature and Dy doping amount. The dielectricproperties were enhanced as the amount of Dy dopingand sintering temperature increases. The sinteringtemperature can be deduced to be related to the particlegrowth of the BT-based ceramic. Also Dy dopingamount can be caused by solubility which affectsoxygen vacancies. Y. Sakabe et al. [100] reported thatDy ion occupied shell regions and grain boundaries.This result can reduce oxygen vacancies which is amajor factor of reducing the electrical properties andreliability of MLCC.

Fig. 2 shows the changes in RC value (Insulationresistance x Capacitance) of BT-based ceramics as afunction of Dy2O3 content and sintering temperature.All samples regardless of Dy2O3 content show similarbehavior. The RC value rapidly increases with sinteringtemperature below 1230 oC. However, RC valuegenerally decreased above 1230 oC. This can be

Fig. 2. Time constants of BT-based ceramics as a function ofDy2O3 content and sintering temperature.

Fig. 3. XRD patterns of different Dy doped BT-based ceramicsFig. 1. Dielectric constant (a) and loss (b) of BT-based ceramics asa function of Dy2O3 contents and sintering temperatures.

Electrical properties of Dy-doped BaTiO3-based Ceramics for MLCC 497

explained by normal or abnormal grain growth of BT-based ceramics that are associated with core-shellstructure [6]. An abnormal grain growth reduces theinsulation resistance factor which is related leakagecurrent of MLCC. Therefore, we can consider anoptimal sintering temperature of 1230 oC owing to highRC value.

Fig. 3 shows the XRD patterns of BT-based ceramicsas a function of Dy2O3 contents sintered at 1,230 oC for2 hours. All the samples show the typical XRD patternsof perovskite structure and there is no second phase.The (110) peak of sample is almost same positionregardless of Dy2O3 contents. This indicated that Dyion (R = 1.03 Å) can be selectively substituted both Ba-site (R = 1.42 Å) and Ti-site (R = 0.61 Å) [3]. Thisphenomenon leads superior reliability as well aselectrical properties of the MLCC. Fig. 4 shows themicrostructure of BT-based ceramics with differentDy2O3 content. Regardless of the amount Dy contents,grain growth of BT-based ceramics show similarbehavior. As compared to the raw grain size (200 nm),the slight grain growth with average grain size of 300nm is occurred. The Dy plays important role ininhibiting grain growth by two reasons. First, Dyexisting in the grain boundary delays the neckingbetween BT grains. The second reason for the delay ofsintering process is an energy consumption which isassociated with doping Dy into BT structure. Thissmall grain size enables the lamellation of dielectricsheets which is one of the most important factors forhigh capacity and reliability MLCC.

Fig. 5(a) shows the temperature coefficient ofcapacitance (TCC) of different Dy doped BT-basedceramics sintered at 1230 oC. The temperaturestability of different Dy doped BT-based ceramics

was conducted in the temperature range from −55to 125 oC. All samples similar behavior, however,the capacitance variations of BT-based ceramicsdoped 0.3, 0.6 wt % Dy doped satisfy X5Rcharacteristics (−55 oC ~ + 85 oC, ± 15%). OtherBT-based ceramics does not meet X5R characteristics.This result is influenced by a stable core-shell structure, aswell as uniform particle growth. Two kinds of BT-based

Fig. 4. SEM images of different Dy doped BT-based ceramics (a)0.3 wt% (b) 0.6 wt% (c) 0.9 wt% (d) 1.2 wt%.

Fig. 5. TCC of BT-based ceramics as a function of Dy2O3 contentsintered at 1230 oC (a), SEM image of fabricated MLCC (b) andMLCC fabricated using 0.6 wt% Dy doped BT-based ceramic (c).

498 Seung-Hwan Lee, Dae-Young Kim, Min-Kee Kim, Hong-Ki Kim, Jeong Hyun Lee, Esther Baek and Jung-Rag Yoon

ceramics doped 0.3 and 0.6 wt% Dy, we selected 0.6 wt%Dy doped BT-based ceramics owing to its superiordielectric properties than that of 0.3 wt%. Fig. 5(b) showsthe SEM image of the fabricated MLCC and Fig. 5(c)shows the capacitance change of MLCC fabricated using0.6 wt% Dy doped BT-based ceramic. The temperaturestability of MLCC further enhanced and it also naturallysatisfied X5R characteristics.

Fig. 6 shows the high temperature insulation resistanceof MLCC (3.2 × 1.6 mm) at 125 oC under stepwiseboosting voltage from 1 Vr ~ 7.5 Vr (1 Vr = 35 V). Thedegradation of insulation resistance was caused by oxygenvacancies which are moved into cathode under DC biasfield stress as time goes by [7, 8]. However, Dy plays acrucial role in delay of oxygen vacancies move and therebypostpone insulation resistance degradation. Generally, theinsulation resistance value decrease less than 10−6 Ω, wecan regard that the reliability of products come to an end. Itcan be seen that fabricated MLCC stably satisfied 6 Vr. This

confirms that high temperature insulation resistance ofMLCC is applicable to the actual products.

Conclusions

The Dy2O3 was added to BT-based ceramics toimprove their dielectric properties. Among them, when0.6 wt% Dy2O3 was added to BT-based ceramicssintered 1230 oC, excellent electrical properties wereobtained with high RC value and stable TCC rangedfrom −55 oC to 125 oC. Especially, the MLCC with0.6 wt% Dy2O3 doped BT-based ceramics shows thatthis composition is a promising MLCC material forsuperior electrical properties with X5R type.

Acknowledgments

The research was support by a grant from theNational Research Foundation (No. 10049864).

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Fig. 6. Elelctrical resistivities at high temperature (125 oC) versustime of MLCC by applying voltage step of DC 1 Vr (1 Vr = 35 V)up to 7.5 Vr